1. Field of the Invention
This invention relates to equipment for transmitting vehicle steering power applied to a steering wheel to the automotive tires, and particularly to such equipment for vehicles equipped with electric-powered steering.
2. Description of the Prior Art
Generally a relatively large force is required to rotate the steering wheel of a vehicle for changing the tire direction when a vehicle stands still or moves at a low speed. Still larger steering power is required for a front wheel drive vehicle, the number of which vehicle has recently substantially increased, since the front part of such vehicle is burdened with a large weight.
Power steering equipment has been known for assisting the steering force of a driver. This equipment is designed to generate a driving force in proportion with the driver's steering force and transmit the generated force to the vehicle steering system. Most of the power steering equipment now in practical use is of the hydraulically powered type. Such hydraulic equipment, comprising control valves and a hydraulic cylinder, generates an auxiliary power force by transferring a quantity of hydraulic oil in accordance with the steering force applied by the driver.
However, the control valves and the hydraulic cylinder are large in size and the pipes and their attachments for interconnecting the valves and the cylinder should be bent to have curvatures larger than certain minimum values to prevent large pressure losses. In addition, such hydraulic equipment requires secure sealing to prevent oil leakage and requires expensive labor in its installation on a vehicle. Therefore, an electric motor has been proposed as a power source for driving power steering equipment.
Installing electric-powered steering equipment on a vehicle involves much different cost and non-restricting conditions according to the method with our invention used to connect the output shaft of the electric motor and the steering mechanism for the reasons to be explained below.
FIG. 1a on the attached drawings shows a general manual steering mechanisms for a vehicle without power steering. In the mechanism, a steering wheel 1 is connected to an upper steering shaft 2, which is further connected to a middle steering shaft 4 via a universal joint 3. The middle steering shaft 4 is then connected through a universal joint 5 to a manual steering gear mechanism 6, which is further connected to tie rods 7 for controlling the direction in which the tires (not shown) are turned.
The manual steering gear mechanism 6 shown in FIG. 1b is of the rack and pinion type. FIG. 1b shows the construction of the steering gear mechanism along with the adjacent components. As shown in FIG. 1b, the output shaft 14 of the universal joint 5 is connected to one end of a lower steering shaft 8, the other end of which is formed with a pinion 9. A rack 10 connected to the tie rods 7 is engaged with the pinion 9. The lower steering shaft 8 is rotatably supported within a manual steering gear housing 12 via bearings 11a and 11b. A dust cover 13 is also generally provided.
With this design, the steering shaft is divided into three parts, namely, upper shaft 2, middle shaft 4 and lower shaft 8, which are connected by two universal joints 3 and 5, thereby resulting in the fact that the axial direction of the pinion 9 cannot be aligned with the axis of the steering wheel 1. In this type of connection, in which the input shaft is obliquely connected with the output shaft, a sinusoidal variation of torque is produced in the output shaft even if a constant torque is applied to the input shaft.
In handling a steering mechanism with such type of connection, the driver may feel heavy or light driving forces at one time or another. Therefore, in such case as in the structure of FIG. 1a, the inclination between the upper and lower steering shafts 2 and 4 is usually designed to be identical with the angle (.theta.) between the middle and lower steering shafts 4 and 8, in order to assure that torque variations at the two points are mutually cancelled.
In power steering equipment in which the rotational speed of the output shaft of an electric motor is reduced, it is preferred to minimize the reduction ratio for simplification of the construction. For this purpose, it is recommended that the output shaft of the power steering should be connected to the input side of the manual steering gear mechanism. In this connection, the output shaft of the power steering equipment may be connected to any one of the upper, middle or lower steering shafts 2, 4 and 8. These three cases will be examined below.
Firstly, attention may be given to the case of connection to the upper steering shaft 2. Since there is a toe or floor board isolating the passenger compartment from the engine compartment in the vicinity of universal joint 3, connection of the upper shaft 2 with the output shaft of the power steering equipment may interfere with vehicle operation by the driver.
Secondly, considering connecting to the middle steering shaft 4, since this shaft 4 is connected to the shaft 8 with an inclination angle .theta. (FIG. 1b), the connection of shaft 4 with the output shaft of the power steering equipment causes the transfer of a variable torque to the steering shaft 8, even if a constant auxiliary torque is generated at the output shaft.
Additionally, since a large portion of the shaft 4 is situated in the engine compartment beyond or outside of the toe or floor board, it is very difficult to add a mechanism to this portion or replace the portion after the vehicle has been completely assembled.
Thirdly, the last case involves connection to the lower steering shaft 8. As shown in FIG. 1a and FIG. 1b, the shaft 8 has such a very limited length that it is difficult to connect the shaft 8 directly to the output shaft of power steering equipment.
Therefore, the possibility arises to provide a longer equivalent of the lower shaft 8 to be connected to the output shaft of the power steering equipment. If this design change is to be done, the inclination between steering shafts 4 and 8 will be changed. In accordance with such a change, the inclination between the upper and middle steering shafts 2 and 4 should be changed. Thus, large scale remodelling will be required of the steering mechanism.